ABI: implement process initialization and shell-timeout trap

Parent: #140 Profiles: #163, #164 Roadmap: #134 Pinned consumer: oven-sh/bun@4982b91e3702094330f3be3883354c52b8c01323 Pinned references: src/jsc/bindings/ZigGlobalObject.cpp:JSCInitialize, src/jsc/bindings/bindings.cpp:JSC__VM__notifyNeedShellTimeoutCheck

Implement the remaining private process-initialization boundary and the distinct shell-timeout VM trap as real engine state, not inert compatibility exports.

Acceptance:

• Implement thread-safe, exactly-once JSCInitialize(env, count, callback, eval_mode, one_shot_startup) with first-call-wins configuration.
• Inspect exactly count nullable sentinel-terminated C-string slots (the pinned ABI precondition), parse BUN_JSC_ entries, and report every unsupported/invalid option through the exact callback slice.
• Apply supported JIT, WebAssembly, SharedArrayBuffer, and ShadowRealm policy to subsequent private VMs; retain eval/one-shot process policy, whose concurrent optimizing-JIT/GC-helper effects are vacuous in zig-js, while explicit specialized context options remain authoritative.
• Make repeated and concurrent initialization calls inert after the winner, without retaining caller memory or invoking later callbacks.
• Implement JSC__VM__notifyNeedShellTimeoutCheck as a thread-safe trap distinct from termination, watchdog, and debugger-break state; consume it at the next execution checkpoint.
• Define the pinned private ABI handle contract: null is inert, each valid live VM targets only its own group, and foreign/stale non-null pointers remain caller UB like the upstream raw-pointer ABI.
• Add independent Home/Bun ABI fixtures plus focused Debug, ReleaseSafe, and TSan coverage.
• Pin exact source/semantic contracts, promote both symbols in both inventories, refresh concise docs/README counts, and reconcile exact main CI.

Implementation is on main in 54298a8 and evidence/docs/CI in 938ef0f. Both exact consumer executables pass in Debug, ReleaseSafe, and TSan. The process fixture covers first-call publication, four blocked concurrent followers, winning-thread reentry, null environment slots, two invalid entries with exact callback slices, later-callback suppression, disabled globals, VM-local one-shot shell-trap consumption, and null notification. Inventories are Home 446/471 and Bun 437/461 with zero unclassified entries. Closure remains gated only on exact main run 29794916349.

The first exact run compiled the new focused fixtures but found three older unit-only CContextGroup literals that did not initialize the newly required process-policy snapshot. Fixed narrowly in 341f387; the filtered unit compile passes 3/3 with zero leaks. Replacement exact main run: https://github.com/zig-utils/zig-js/actions/runs/29795137428.

Complete. Process initialization and the distinct shell-timeout trap are pinned, implemented, inventoried, documented, and independently exercised for Home/Bun in Debug, ReleaseSafe, and TSan. Exact main run 29801586207 passed all 43 jobs.